NiMH Battery Dead (already?)

[michealcollins]

So I bought a 6V NiMH rechargeable battery for the SOR Voice Controlled Robot instructible. I have not even gotten to use it on the servos and it's already reading about 4 V. All I have used it for is powering up the Axon II microcontroller while programming. I had to switch it on and off a bunch of times today because I was troubleshooting some things I had hooked up to it (FDTI board, VR module).

I noticed I didnt have anything on the Axon II LED display...so I measured the battery with a multimeter, and it read about 3.99V. As far as I'm concerned, that's a dead battery. Is that normal? I figured I would get a lot of use from it before it died.

Anyway, I dont have a charger either....so can anyone recommend a decent, inexpensive charger? Thanx 

Mike

[michealcollins]

Ok, no charger needed. I used a power supply at my school and it worked fine. My professor approved as well lol.

But I would like to know how long this thing is supposed to last before charging. Seemed I hardly got any use out of it.

Mike

[joe61]

They don't come charged typically. You need to charge them before first use. I don't know what your prof recommended but you probably should get a charger made for the pack. Definitely don't try something like that with lipo batteries.

[Soeren]

Hi,

What did the battery measure when charged?

Measure each cell of the battery to be sure you haven't got a dead cell in the pack or uneven charge in general.


It's impossible to tell how long it should last, as you didn't mention, neither its capacity, nor how hard you loaded it.

[michealcollins]

It's a 3300 mAh side by side NiMH battery with a Hitec connector. Not sure how hard I loaded it...I just hooked it into my Axon II microcontroller for the basic upload of the hardware programs. It was never on for more than 5 minutes at a time. I hadnt even gotten a chance to run the servos (2) for the robot.

But anyway, my professor got his doctorate in wireless and telecommunications...he seemed to be confident that I wasnt going to hurt the battery by using the lab power supply at a low ampere setting (.5-.8 amperes). It worked pretty good actually.

Anyway, its just sounding like I need to charge it up before use. Thanks!

[joe61]

But anyway, my professor got his doctorate in wireless and telecommunications...he seemed to be confident that I wasnt going to hurt the battery by using the lab power supply at a low ampere setting (.5-.8 amperes). It worked pretty good actually.

NiMH batteries are one thing, but if you go to Li-Po or similar, use a charger specifically meant for the type of battery pack. You could have quite an exciting time otherwise.

[Gertlex]

What others have said seems accurate to me.  I can recommend a charger that's good for most all Lipo/NiMH batteris (as long as you have a the given connector/adapter).  I use one of these: http://www.amazon.com/Titan-Digital-Battery-Balance-Charger/dp/B005HTOPTG  I got mine on ebay with power supply bought separately for about $30 total.  Good investment in my case, as I shouldn't need another battery charger "ever".

[Soeren]

Hi,

It's a 3300 mAh side by side NiMH battery with a Hitec connector. Not sure how hard I loaded it...I just hooked it into my Axon II microcontroller for the basic upload of the hardware programs. It was never on for more than 5 minutes at a time. I hadnt even gotten a chance to run the servos (2) for the robot.

But anyway, my professor got his doctorate in wireless and telecommunications...he seemed to be confident that I wasnt going to hurt the battery by using the lab power supply at a low ampere setting (.5-.8 amperes). It worked pretty good actually.

800mA is way to high a current for charging this way!

You won't hurt the battery provided you do the following...
You set the PSU to max. 1.45V/cell.
If the battery is really flat, you set the current limiter to max. C100 for starters (that's 33mA in your case) and charge until all cells are at minimum 1.1V/cell (1.2V/cell is better).
When the battery have reached this minimum, you can set the current limiter to C10 (i.e. 330mAh) and charge until the current tapers off to around C100 (33mA) - that should take around 14..16 hours for a completely flat battery.
Remove the charge current within the next couple of days.

This will safely charge the battery, as long as you keep to the exact voltage setting and don't try charging very flat batteries at a high current.

If you get different voltage readings for the cells in a battery, you need an equalizing charge, where you charge with C100 max (C200 is even better, if you can wait that long) and charge them untill they all show exactly the same voltage +/- 1mV.

The shorter the wires from the PSU to the battery and the firmer the terminals are gripped, the better charge.


But it get boring real soon. Getting a good quality charger with fast charge and multiple charge termination techniques, you could charge the same in 2..3 hours without having to remember to disconnect.
And the best of them checks if there's a need for equalization and applies it automatically.

[michealcollins]

Thank you so much for the info! Im not sure the PSU we have at school has current limiter functions...but I took your advice (the best I could) and set the voltage for 6V (about 1.2 V per cell) and the current for 33 mA. That's about all I can do with this particular PSU.

I already charged this thing for like 15 minutes a few times at the previous 800 mA...do you think I damaged it already?

Mike

[Soeren]

Hi,

Thank you so much for the info! Im not sure the PSU we have at school has current limiter functions...but I took your advice (the best I could) and set the voltage for 6V (about 1.2 V per cell) and the current for 33 mA. That's about all I can do with this particular PSU.

Remember, after a short while it will be above 1.1V/cell and then you can crank it up to 330mAh (don't chase the last digit though - somewhere between 320mA and 350mA will be more than fine).
If you keep it at 1.2V/cell, you'll never get it charged fully. Get it to 1.4V/cell (but NOT above 1.45V/cell)
For a 5-cell battery, this will be 7.00V to 7.25V
Your lab supply do have a current limiter -  it's the knob that you used to set it to eg. 33mA

At 7V and 330mA, you should leave it on for 14-16 hours from flat (an extra day won't hurt anything, but won't help either), to get it fully charged. Only partially charging it each time will lead to voltage depression and then the cell voltage will appear lower in use.

I already charged this thing for like 15 minutes a few times at the previous 800 mA...do you think I damaged it already?

Not in a measurable way I'd guess, but capacity may have taken a very small hit.

[michealcollins]

Well to be safe, I'm just gonna bite the bullet and purchase a charger. Thanks so much for the help mate! Cheers!

Mike

[Soeren]

Hi,

You'll soon be glad you did

[Admin]

Just two side notes . . .

The old and el-cheapo NiMH batteries have a high self discharge rate. It means that if you let it sit there for a week or few, it'll run out of charge on it's on. The newer quality NiMHs have a very slow self-discharge rate, taking many months. Only the quality batteries will tell you the rates in the battery datasheet.

Also, servos drain power even when you don't use them. Lets say you have a battery and servos plugged into your Axon, and your Axon isn't programmed to do anything. The servos will still happily drain your battery dry over a period of a few days.

This may or may not be relevant to your situation, but your other posts suggest you've been working on this for ~2 months without recharging

[michealcollins]

Yeah I bought the good one ($22 Tenergy) but never charged it before use. I was in fact using it without recharge, but I figured my use was in such short durations that it wouldnt drain. Boy was I wrong. I bought the charger, and now she is working great. Thanks for the advice fellas!

Mike